1. Field of the Invention
The present invention relates to an electron multiplying solid-state image pickup apparatus, a method and a computer program product for adjusting an electron multiplying gain.
2. Description of the Related Art
An electron multiplying solid-state image pickup device is well known as a solid-state image pickup device with high sensitivity. For example, Japanese Patent Application Laid-Open No. 2006-203222 and Japanese Patent Application Laid-Open No. 2007-274050 disclose the electron multiplying solid-state image pickup device which multiplies electrons using impact ionization. Here, the impact ionization means that electrons accelerated by an electric field collide with a crystal lattice to generate a pair of an electron and a hole.
FIG. 8 shows a basic operation of an electron multiplying CCD (Charged Coupled Device) image pickup device using the impact ionization, which is disclosed in Japanese Patent Application Laid-Open No. 2007-274050. The electron multiplying CCD image pickup device 100 includes an image pickup area 103 including a photodiode 101 and a vertical CCD register 102, and a storage area 104 which accumulates signal charge transferred from the image pickup area 103. A horizontal CCD register 105 receives the signal charge accumulated in the storage area 104 and transfers the signal charge in a horizontal direction. An electron multiplying CCD register 106 connects with the horizontal CCD register 105, and multiplies the transferred signal charge. An output amplifier 107 converts the signal charge amplified by the electron multiplying CCD register 106 into a voltage signal 108. The voltage signal 108 is outputted from an output terminal 109 to the outside.
FIG. 9 shows a cross section and an electric potential distribution profile of the electron multiplying CCD register 106 shown in FIG. 8. The electron multiplying CCD register 106 includes a gate oxide film 121. A first horizontal transfer gate electrode 122, a second horizontal transfer gate electrode 123 and a electron multiplying gate 124 are separately formed on a top surface of the gate oxide film 121 at a predetermined intervals.
Hereinafter, operations of the electron multiplying CCD image pickup device 100 will be described. The photodiode 101 generates signal charge 141 through photoelectric conversion. The vertical CCD register 102 read the signal charge 141. The signal charge 141 read by the vertical CCD register 102 is vertically transferred from the image pickup area 103 to the storage area 104 and reaches the horizontal register 105. The signal charge 141 is further horizontally transferred through the horizontal CCD register 105 to the electron multiplying CCD register 106. The electron multiplying CCD register 106 includes a large potential difference (high electric field) 131 which is generated due to applying high voltage to the electron multiplying gate electrode 124. An electron as the signal charge 141 is accelerated due to the large potential difference 131 and collides with a silicon crystal lattice. The collision causes the impact ionization to generate a new pair of an electron 144 and a hole 145. Then, the hole 145 of the generated pair flows to a silicon substrate to disappear. The electron 144 is captured in a potential well. Several hundred electron multiplying gate electrodes 124 are provided and processes described above are repeated at each of the several hundred electron multiplying gate electrodes 124. As a result of the repeated processes, it is possible to multiply the signal charge 141.
FIG. 10 shows a relation between electron multiplying gate voltage and an electron multiplying gain of the electron multiplying CCD image pickup device 100 which carries out the above mentioned operations. A characteristic curve 161 in FIG. 10 indicates that an electron multiplying gain becomes high as the voltage applied to the electron multiplying gate electrodes 124 becomes high. That is, the electron multiplying gain is determined based on the voltage applied to the electron multiplying gate electrodes 124.
Further, Japanese Patent Application Laid-Open No. 2000-324502 discloses an image pickup apparatus which calculates information on aging of color characteristic of the image pickup apparatus, based on calibrated color characteristics and stored initial color characteristics, and then processes an image signal according to the information on the aging. Japanese Patent Application Laid-Open No. 1994-233332 discloses a signal generating apparatus which generates a test signal for testing a signal processing circuit of CCD camera.